Method of preparing copper plating layer having high adhesion to magnesium alloy using electroplating

ABSTRACT

Disclosed is a method of preparing a copper electroplating layer having high adhesion to a magnesium alloy, which is advantageous because the usability of the magnesium alloy, having the highest specific strength among actually usable metals, can be increased through the development of a process of forming a uniform copper plating layer upon electroplating of the magnesium alloy. The method of preparing a copper electroplating layer having high adhesion to a magnesium alloy of this invention is characterized in that the magnesium alloy is pretreated with a plating pretreatment solution to form a film for electroplating, serving as a magnesium alloy pretreatment layer, exhibiting a uniform current distribution, which is then electroplated with copper to form the copper plating layer. According to this invention, through the pretreatment of the magnesium alloy, the adhesion of the copper plating layer to the film for electroplating formed on the magnesium alloy can be increased.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates, generally, to a pretreatment method of amagnesium alloy for electroplating the magnesium alloy, and moreparticularly, to a method of pretreating a magnesium alloy toelectroplate the magnesium alloy so as to increase the usability of themagnesium alloy, having the highest specific strength among actuallyusable metals, through the development of a magnesium pretreatmentprocess for the formation of a uniform copper (Cu) electroplating layeron the magnesium alloy.

2. Description of the Related Art

In general, a magnesium (Mg) alloy, which has the smallest weight amongactually usable metals, has excellent specific strength (specificgravity/strength) and easy processability, and is thus widely used forparts of automobiles, computers, or information communicationapparatuses. Although the magnesium alloy has been prepared mainly usinga die casting process, an extrusion process, a rolling process, etc., itis recently formed using a thixo-molding process as an advancedtechnique by a combination of metal die casting and plastic injectionmolding. With the development of magnesium alloys able to undergo pressforming, the demand thereof will increase more and more.

However, since the magnesium alloy has a relatively low standardpotential among the actually usable metals, it may be easily oxidized inair, thus having corrosion resistance insufficient for use as anactually usable metal. Thus, great efforts have been made to increasethe corrosion resistance of the magnesium alloy.

As surface treatment techniques for improvements in corrosion resistanceof the magnesium alloy, a chromate treatment process has been widelyconducted. However, the chromate treatment suffers because it discolorsthe surface of magnesium and a chromium compound of a solution used forchromate treatment causes environmental problems, and thus the usethereof is limited according to international environmentalrestrictions.

Therefore, although the development of non-chromate treatment has beenactively conducted in recent years, such non-chromate treatment resultsin lower corrosion resistance and higher expense than those of theconventional chromate treatment.

In addition, an anodizing process has been developed, but it islimitedly used for internal parts where external appearance is notregarded as important or is applied only to under-films of coating orpainting.

As the other surface treatment for an increase in corrosion resistanceof the magnesium alloy, techniques for plating a surface of a magnesiumalloy using a dry or wet process are proposed. However, the magnesiumalloy is difficult to dry plate, including deposition plating in avacuum, due to the high vapor pressure thereof.

The wet plating techniques are classified into a wet electroplatingprocess using electrical energy, and an electroless plating processusing a chemical reaction. As such, the electropless plating process isexemplified by an electroless nickel plating process. However, theelectroless nickel plating process is disadvantageous because anelectroless nickel plating solution has high production cost, and aswell an electroless nickel plating layer should be double-, triple- orquadruple-formed while varying the amounts of phosphorus (P) to increasethe corrosion resistance of magnesium.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made keeping in mind theabove problems occurring in the related art, and an object of thepresent invention is to provide a method of forming a copper platinglayer having high adhesion to a magnesium alloy through electroplating,in which a film for electroplating is formed on the magnesium alloy andthen copper (Cu) plating is conducted, such that the magnesium alloy,which is susceptible to an acid, in particular, an aqueous sodiumchloride solution, can have high corrosion resistance, thereforeresulting in increased usability of the magnesium alloy.

Further, the present invention, aiming to be a method of preparing acopper electroplating layer having high adhesion to a magnesium alloy,is characterized in that the magnesium alloy is pretreated with aplating pretreatment solution to form a film for electroplating, servingas a magnesium alloy pretreatment layer, exhibiting a uniform currentdistribution, which is then electroplated with copper to form the copperplating layer.

In order to accomplish the above object, the present invention providesa method of preparing a copper plating layer having high adhesion to amagnesium alloy through electroplating, comprising pretreating themagnesium alloy with a plating pretreatment solution to form a film forelectroplating, serving as a magnesium alloy pretreatment layer,exhibiting a uniform current distribution; and conducting copperelectroplating on the magnesium alloy treatment layer to form the copperplating layer firmly adhering to the magnesium alloy pretreatment layer,in which, upon separation of the copper plating layer by force, thesurface of the magnesium alloy adhering to the copper plating layerexhibits coarse grains contained in the pretreatment layer.

In addition, in the method of the present invention, the platingpretreatment solution may comprise 5˜130 g/l of ZnSO₄, 30˜450 g/l ofNa₄P₂O₇, 4˜100 g/l of KF, and 2˜100 g/l of Na₂CO₃.

In addition, in the method of the present invention, each of K₄P₂O₇ andNa₂CO₃ may be used in an amount of about 5˜20 vol % based on a volume ofa solution of a dry bath when chemical components of the platingpretreatment solution have fatigue due to frequent plating, in order tocontinuously maintain adhesion between the copper plating layer andmagnesium alloy.

In addition, in the method of the present invention, the platingpretreatment solution may comprise 4˜145 g/l of ZnSO₄, 15˜450 g/l ofNa₄P₂O₇, 1˜125 g/l of NaF, 1˜125 g/l of Na₂CO₃ and 0.5˜45 g/l ofKNaC₄H₄O₆, with additives.

In addition, in the method of the present invention, the platingpretreatment solution may comprise 5˜80 g/l of ZnSO₄, 4˜380 g/l ofK₄P₂O₇, 5˜80 g/l of KF, and 2˜120 g/l of Na₂CO₃.

In addition, in the method of the present invention, the platingpretreatment solution may comprise 7˜220 g/l of ZnSO₄, 45˜600 g/l ofK₄P₂O₇, 3˜100 g/l of KF, 2˜130 g/l of Na₂CO₃, and 0.5˜58 g/l ofKNaC₄H₄O₆, with additives.

In addition, in the method of the present invention, the copper platinglayer may be formed by sequentially conducting first copper cyanideplating and second copper pyrophosphate (CuP₂O₇) plating or third coppersulfate plating, on the magnesium alloy pretreatment layer.

In addition, in the method of the present invention, the KNaC₄H₄O₆,added to continuously maintain the adhesion among the components of theplating pretreatment solution, may be used in an amount of 10 vol % orless, due to a sensitive substitution reaction, based on the volume ofthe solution of the dry bath.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be more clearly understood from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a cross-sectional view showing a copper plating layer formedusing a process of preparing a copper plating layer having high adhesionto a magnesium alloy through electroplating, according to the presentinvention;

FIG. 2 is a view showing the process of preparing a copper plating layerhaving high adhesion to a magnesium alloy through electroplating,according to the present invention;

FIG. 3 is a photograph showing a state of a copper plating layer beingforcibly separated from a magnesium sample, which is pretreated and thenplated with copper according to a conventional technique (usingconditions other than the conditions of the present invention);

FIG. 4 is a photograph showing a state of the copper plating layer beingforcibly separated from a magnesium sample, which is pretreated and thenplated with copper according to the present invention;

FIG. 5 is a 60-times-magnified photograph showing the surface ofmagnesium, which is pretreated according to the conventional technique(using conditions other than the conditions of the present invention);

FIG. 6 is a 200-times-magnified photograph showing the surface ofmagnesium, which is pretreated according to the conventional technique(using conditions other than the conditions of the present invention);

FIG. 7 is a 60-times-magnified photograph showing the surface ofmagnesium, which is pretreated according to the present invention;

FIG. 8 is a 200-times-magnified photograph showing the surface ofmagnesium, which is pretreated according to the present invention;

FIG. 9 is a 60-times-magnified photograph showing the pretreated surfaceof magnesium, resulting from forcibly separating the copper platinglayer from the magnesium sample, which is pretreated and then platedwith copper according to the conventional technique (using conditionsother than the conditions of the present invention);

FIG. 10 is a 200-times-magnified photograph showing the pretreatedsurface of magnesium, resulting from forcibly separating the copperplating layer from the magnesium sample, which is pretreated and thenplated with copper according to the conventional technique (usingconditions other than the conditions of the present invention);

FIG. 11 is a 60-times-magnified photograph showing the pretreatedsurface of magnesium, resulting from forcibly separating the copperplating layer from the magnesium sample, which is pretreated and thenplated with copper according to the present invention;

FIG. 12 is a 200-times-magnified photograph showing the pretreatedsurface of magnesium, resulting from forcibly separating the copperplating layer from the magnesium sample, which is pretreated and thenplated with copper according to the present invention;

FIG. 13 is a 60-times-magnified photograph showing the surface of thecopper plating layer, resulting from forcibly separating the copperplating layer from the magnesium sample, which is pretreated and thenplated with copper according to the conventional technique (usingconditions other than the conditions of the present invention);

FIG. 14 is a 200-times-magnified photograph showing the surface of thecopper plating layer, resulting from forcibly separating the copperplating layer from the magnesium sample, which is pretreated and thenplated with copper according to the conventional technique (usingconditions other than the conditions of the present invention);

FIG. 15 is a 60-times-magnified photograph showing the surface of thecopper plating layer, resulting from forcibly separating the copperplating layer from the magnesium sample, which is pretreated and thenplated with copper according to the present invention; and

FIG. 16 is a 200-times-magnified photograph showing the surface of thecopper plating layer, resulting from forcibly separating the copperplating layer from the magnesium sample, which is pretreated and thenplated with copper according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a detailed description will be given of a method ofpreparing a copper plating layer having high adhesion to a magnesiumalloy through electroplating according to the present invention, withreference to the appended drawings.

FIG. 1 is a cross-sectional view showing a copper plating layer formedthrough electroplating according to a method of pretreating themagnesium alloy for electroplating of the magnesium alloy of the presentinvention, and FIG. 2 is a view sequentially showing a process ofpreparing a copper plating layer according to the method of pretreatingthe magnesium alloy for electroplating of the magnesium alloy of thepresent invention. FIG. 3 is a photograph showing the state of thecopper plating layer being forcibly separated from a magnesium sample,which is pretreated and then plated with copper according to aconventional technique (using conditions other than the conditions ofthe present invention), and FIG. 4 is a photograph showing the state ofthe copper plating layer being forcibly separated from a magnesiumsample, which is pretreated and then plated with copper under theconditions of the present invention. FIG. 5 is a 60-times-magnifiedphotograph showing the surface of magnesium, which is pretreatedaccording to the conventional technique (using conditions other than theconditions of the present invention), FIG. 6 is a 200-times-magnifiedphotograph showing the surface of magnesium, which is pretreatedaccording to the conventional technique (using conditions other than theconditions of the present invention), FIG. 7 is a 60-times-magnifiedphotograph showing the surface of magnesium, which is pretreated underthe conditions of the present invention, and FIG. 8 is a200-times-magnified photograph showing the surface of magnesium, whichis pretreated under the conditions of the present invention. FIG. 9 is a60-times-magnified photograph showing the pretreated surface ofmagnesium, resulting from forcibly separating the copper plating layerfrom the magnesium sample, which is pretreated and then plated withcopper according to the conventional technique (using conditions otherthan the conditions of the present invention), FIG. 10 is a200-times-magnified photograph showing the pretreated surface ofmagnesium, resulting from forcibly separating the copper plating layerfrom the magnesium sample, which is pretreated and then plated withcopper according to the conventional technique (using conditions otherthan the conditions of the present invention), FIG. 11 is a60-times-magnified photograph showing the pretreated surface ofmagnesium, resulting from forcibly separating the copper plating layerfrom the magnesium sample, which is pretreated and then plated withcopper under the conditions of the present invention, and FIG. 12 is a200-times-magnified photograph showing the pretreated surface ofmagnesium, resulting from forcibly separating the copper plating layerfrom the magnesium sample, which is pretreated and then plated withcopper under the conditions of the present invention. FIG. 13 is a60-times-magnified photograph showing the surface of the copper platinglayer, resulting from forcibly separating the copper plating layer fromthe magnesium sample, which is pretreated and then plated with copperaccording to the conventional technique (using conditions other than theconditions of the present invention), FIG. 14 is a 200-times-magnifiedphotograph showing the surface of the copper plating layer, resultingfrom forcibly separating the copper plating layer from the magnesiumsample, which is pretreated and then plated with copper according to theconventional technique (using conditions other than the conditions ofthe present invention), FIG. 15 is a 60-times-magnified photographshowing the surface of the copper plating layer, resulting from forciblyseparating the copper plating layer from the magnesium sample, which ispretreated and then plated with copper under the conditions of thepresent invention, and FIG. 16 is a 200-times-magnified photographshowing the surface of the copper plating layer, resulting from forciblyseparating the copper plating layer from the magnesium sample, which ispretreated and then plated with copper under the conditions of thepresent invention.

As shown in FIG. 1, a magnesium alloy sheet can be directly plated withcopper (Cu) using a wet process. Since the magnesium alloy is highlycorroded by an acid, it is difficult to plate. Further, the magnesiumalloy is very sensitive to a pretreatment process (degreasing, acidwashing, activation), as well as the copper plating process.

Of the pretreatment, the activation process greatly affects adhesion anduniformity of a copper plating layer to be subsequently formed. Thus, inorder to form a uniform copper plating layer highly adhering to themagnesium alloy, the pretreatment process must be precisely conductedalong with the use of a certain copper plating solution. That is, awater washing process must be thoroughly conducted at each step.Otherwise, the pre-process solution mixed with a subsequent processsolution hinders an electrochemical plating process, thus undesirablycausing poor plating properties.

Unlike a conventional copper plating solution, in the present invention,a copper plating solution for use in formation of a copper platinglayer, having high adhesion to the magnesium alloy, comprises a weakacidic aqueous solution composed mainly of copper cyanide, sodiumcyanide, copper sulfate, and sulfuric acid with additives. Using such anaqueous solution, the surface of the magnesium alloy is wet plated withcopper. As such, the shape of the magnesium alloy is not limited.

As shown in FIG. 2, illustrating the copper plating process according tothe method of pretreating the magnesium alloy for electroplating of themagnesium alloy of the present invention, the magnesium alloy isprocessed using a die casting process, degreased, etched to increase theadhesion, and then pretreated for copper plating. The pretreatmentprocess of the magnesium alloy is very important for conducting thecopper plating process on the magnesium alloy. The copper platingprocess includes two or three plating steps to form a desired copperplating layer.

As shown in FIG. 3 which is a photograph showing the state of the copperplating layer being forcibly separated from the magnesium sample, whichis pretreated and then plated with copper according to the conventionaltechnique (using conditions other than the conditions of the presentinvention), the magnesium alloy sheet (having a pretreatment layerformed on the surface thereof) plated with copper is torn, so that thecopper plating layer is forcibly separated from the magnesium alloysheet. As such, due to the low adhesion between the copper plating layerand the magnesium alloy pretreatment layer, a considerably large portionof the copper plating layer is removed from the magnesium alloypretreatment layer. This is because the pretreatment process isconducted under conditions other than the conditions of the presentinvention, resulting in remarkably low adhesion between the magnesiumalloy and the copper plating layer.

As shown in FIG. 4, which is a photograph showing the state of thecopper plating layer being forcibly separated from the magnesium sample,which is pretreated and then plated with copper under the conditions ofthe present invention, although the magnesium alloy sheet is torn alongwith the copper plating layer, the copper plating layer has difficultyin being separated from the magnesium alloy pretreatment layer. This isbecause the pretreatment process conducted under the conditions of thepresent invention results in greatly increased adhesion.

FIG. 5 is a 60-times-magnified photograph showing the surface of themagnesium alloy pretreatment layer according to the conventionaltechnique (using conditions other than the conditions of the presentinvention), and FIG. 6 is a 200-times-magnified photograph showing thesurface of the magnesium alloy pretreatment layer according to theconventional technique (using conditions other than the conditions ofthe present invention). As shown in these drawings, many pinholes may beformed in the surface, and the state of the surface is poor due to thepresence of impurities. Since such pinholes may be formed by aninsufficient water washing process or inappropriate bath conditions, thegeneration thereof may be prevented using appropriate plating conditionsand additives.

FIG. 7 is a 60-times-magnified photograph showing the surface of themagnesium alloy pretreatment layer formed under the conditions of thepresent invention, and FIG. 8 is a 200-times-magnified photographshowing the surface of the magnesium alloy pretreatment layer formedunder the conditions of the present invention. As is apparent from thesedrawings, pinholes, which have been seen in FIGS. 5 and 6, are notfound. However, all the pretreatment layers of FIGS. 5 to 8 areconfirmed to have similar surface roughness.

FIG. 9 is a 60-times-magnified photograph showing the surface of themagnesium alloy pretreatment layer after the copper plating layer isforcibly removed from the magnesium sample, which is pretreated and thenplated with copper according to the conventional technique (usingconditions other than the conditions of the present invention), and FIG.10 is a 200-times-magnified photograph showing the pretreated surface ofthe magnesium alloy pretreatment layer after the copper plating layer isforcibly removed from the magnesium sample, which is pretreated and thenplated with copper according to the conventional technique (usingconditions other than the conditions of the present invention). In FIG.9, a plurality of pinholes is found in the magnesium alloy pretreatmentlayer, and there is no evidence that the copper plating layer firmlyadheres to the magnesium alloy pretreatment layer. In addition, yellowcopper, present on the surface of the magnesium alloy pretreatmentlayer, indicates that copper oxide is formed attributable to theinappropriate plating bath conditions. Such copper oxide has noconductivity and thus functions to hinder the plating process, andtherefore the plating bath conditions must be carefully controlled.Consequently, it appears that the plating layer does not firmly adhereto the magnesium alloy pretreatment layer. If the copper plating layerfirmly adheres to the magnesium alloy pretreatment layer, such amagnesium alloy layer should be separated along with the copper platinglayer upon removal of the copper plating layer. However, as seen inFIGS. 9 and 10, the magnesium alloy layer, which is separated along withthe copper plating layer as it is attached to the copper plating layer,is only slightly observed. Thus, it is believed that an adhering processis not conducted as desired.

FIG. 11 is a 60-times-magnified photograph showing the surface of themagnesium alloy pretreatment layer after the copper plating layer isforcibly removed from the magnesium sample, which is pretreated and thenplated with copper under the conditions of the present invention, andFIG. 12 is a 200-times-magnified photograph showing the surface of themagnesium alloy pretreatment layer after the copper plating layer isforcibly removed from the magnesium sample, which is pretreated and thenplated with copper under the conditions of the present invention. Fromthe results of FIGS. 11 and 12, upon the forced separation of the copperplating layer, because there is high adhesion between the copper platinglayer and the magnesium alloy pretreatment layer and the magnesium alloypretreatment layer is relatively weaker than the copper plating layer,the separation is generated along through the magnesium alloypretreatment layer. Furthermore, in FIGS. 9 and 11, relative strengthsof force, applied to compulsively separate the copper plating layer, aregreatly different from each other, which can be confirmed from thesurface of the separated magnesium alloy layer. Although the magnesiumalloy pretreatment layers of FIGS. 5 to 8 have similar surface roughnesswith the exception of some pinholes and defect rates, the magnesiumalloy layers of FIGS. 9 to 11 have different surface roughness. That is,large crystal grains are observed in FIGS. 11 and 12, whereas they arenot observed in FIGS. 9 and 10. This is because when the copper platinglayer firmly adheres to the magnesium alloy layer through thepretreatment of the present invention, Na₂Cu(CN)₃ of the copper platinglayer, having high affinity to the magnesium alloy, is formed on thesurface of the pretreatment layer, thus exhibiting excellent adhesion.In addition, the reason why the large crystal grains protrude is thatthe grain size of Na₂Cu(CN)₃ is large and the separation is generatedalong through the pretreatment layer, thereby exposing the surfaces ofthe grains of Na₂Cu(CN)₃, upon the separation of the copper platinglayer by force.

FIG. 13 is a 60-times-magnified photograph showing the surface of thecopper plating layer after being forcibly removed from the magnesiumsample, which is pretreated and then plated with copper according to theconventional technique (using conditions other than the conditions ofthe present invention), and FIG. 14 is a 200-times-magnified photographshowing the surface of the copper plating layer after being forciblyremoved from the magnesium sample, which is pretreated and then platedwith copper according to the conventional technique (using conditionsother than the conditions of the present invention). As shown in theenlarged surface of the copper plating layer that has been attached tothe pretreated magnesium alloy, there is no attachment of the magnesiumalloy layer to the copper plating layer from the judgment of the exposedcopper plating layer. Hence, it is confirmed that the copper platinglayer has low adhesion to the magnesium alloy.

FIG. 15 is a 60-times-magnified photograph showing the surface of thecopper plating layer after being forcibly removed from the magnesiumsample, which is pretreated and then plated with copper under theconditions of the present invention, and FIG. 16 is a200-times-magnified photograph showing the surface of the copper platinglayer after being forcibly removed from the magnesium sample, which ispretreated and then plated with copper under the conditions of thepresent invention. Compared to the photographs shown in FIGS. 13 and 14,from the results of FIGS. 15 and 16, it can be seen that the magnesiumalloy layer is attached to almost the entire surface of the copperplating layer. As such, the surface of the magnesium alloy adhering tothe copper plating layer forcibly separated exhibits large grainscontained in the pretreatment layer. This indicates that the pretreatedmagnesium alloy layer is removed from the magnesium alloy sheet.

A better understanding of the present invention may be obtained in lightof the following examples, which are set forth to illustrate, but arenot to be construed to limit the present invention.

EXAMPLE 1

A magnesium alloy was processed through die casting, dipped into adegreasing solution at 30˜90° C., allowed to stand in the solution atabout 10 pH for 10 min to remove all oil components, and then washedwith water to completely eliminate the degreasing solution component. Assuch, if a very small amount of the degreasing solution componentremains, an electrochemical reaction rate is decreased upon plating,thus causing undesirable expansion of the surface and formation ofpinholes, resulting in decreased adhesion between a base metal and aplating layer. Thus, thorough water washing must be conducted.

An aqueous solution having the composition shown in Table 1 below wasprepared, the temperature thereof was adjusted, and a dipping processwas conducted using such an aqueous solution. The temperature of theaqueous solution, the dipping time and the pH are given in Table 1below. TABLE 1 Composition of Temp. of Aqueous Dipping Time AqueousSolution Solution (° C.) (min) pH ZnSO₄ + Na₄P₂O₇ + 30˜90 1˜20 1˜14 KF +Na₂CO3

In addition, an aqueous solution having the composition shown in Table 2below was prepared, the temperature thereof was adjusted, and a dippingprocess was conducted using the aqueous solution. The temperature ofaqueous solution, the dipping time and the pH are given in Table 2below. TABLE 2 Composition of Temp. of Aqueous Dipping Time AqueousSolution Solution (° C.) (min) pH ZnSO₄ + K₄P₂O₇ + 20˜90 1˜20 0.5˜14NaF + Na₂CO₃ + KNaC₄H₄O₆

In addition, an aqueous solution having the composition shown in Table 3below was prepared, the temperature thereof was adjusted, and a dippingprocess was conducted in the aqueous solution. The temperature ofaqueous solution, the dipping time and the pH are given in Table 3below. TABLE 3 Composition of Temp. of Aqueous Dipping Time AqueousSolution Solution (° C.) (min) pH ZnSO₄ + K₄P₂O₇ + KF + 18˜90 1˜200.3˜14 Na₂CO₃

When the chemical components used had fatigue due to frequent plating,each of K₄P₂O₇ and Na₂CO₃ was added in an amount of about 5˜20 vol %based on the volume of the solution of a dry bath so as to continuouslymaintain adhesion.

Since the magnesium alloy, which is a composite material, is verysensitive to the copper plating process, the magnesium alloy must bepretreated under the conditions of the present invention.

As such, it should be noted that KNaC₄H₄O₆ causes a sensitivesubstitution reaction even though it is added in a very small amount,and thus should be used in an amount not higher than 10 vol % based onthe volume of the solution of the bath.

In the present invention, upon electroplating of the magnesium alloy,the pretreatment solution is formed to have NaF instead of KF, andK₄P₂O₇ instead of Na₄P₂O₇, with a small amount of KNaC₄H₄O₆, and therebythe copper plating layer may have high adhesion even though the chemicalcomponents in the dry bath have fatigue.

The magnesium alloy having a film thereon through the platingpretreatment conditions shown in Tables 1 to 3 is electroplated to forma copper plating layer. In addition, before the plating pretreatment, awater bath at 80˜90° C. may be applied depending on the properties ofproducts, and thus the plating pretreatment time may be shortened.

Upon copper plating, copper cyanide plating is first conducted toincrease adhesion of a base metal. Using the following aqueous solution,temperature, voltage, current and conductive time are controlled to forma copper cyanide plating layer.

The copper cyanide plating process is conducted to firmly attach thecopper cyanide plating layer to the magnesium alloy pretreatment layer.Thus, the copper cyanide (Na₂Cu(CN)₃) plating layer is formed tosecurely adhere to the magnesium alloy pretreatment layer. Temp. ofComposition of Aqueous Aqueous Solution Voltage Current ConductingSolution (° C.) (V) (A/dm2) Time (min) pH CuCN + NaCN + 25˜35 2˜4 3˜51˜5 9˜10 Na₂CO₃

After the copper cyanide plating layer is formed, copper pyrophosphate(CuP₂O₇) plating and then copper sulfate plating may be selectivelyconducted to remove pinholes.

Since the copper cyanide plating layer is formed on the rough surface ofthe magnesium alloy having many pinholes, copper pyrophosphate (CuP₂O₇)plating is conducted, in order to fill the pinholes and flatten thesurface. Further, the sulfate copper plating may be selectivelyconducted to fill the pinholes and flatten he surface.

The copper cyanide grains are very large and rough, which can beindirectly confirmed from the photograph of the separated magnesiumalloy pretreatment layer as in FIG. 12.

The copper sulfate plating is conducted using the following aqueoussolution while controlling the temperature, voltage, current, andconductive time, to form a copper sulfate plating layer. Temp. ofComposition of Aqueous Aqueous Solution Voltage Current ConductingSolution (° C.) (V) (A/dm²) Time (min) pH CuSO₄ + H₂SO₄ + 30˜50 4˜6 5˜81˜5 9˜10 Chlorine Ion + Na₂CO₃

Therefore, the copper plating process actually includes two or threesteps, in which copper cyanide plating is first conducted on thepretreated surface of the magnesium alloy and then selectively, copperpyrophosphate (CuP₂O₇) plating and then copper sulfate plating may beconducted. TABLE 4 Adhesion File Tape Pencil Lead Sample Test Test Test(H) Ex. No. 1 ◯ ◯ 4 2 ◯ ◯ 4 3 ◯ ◯ 4 4 ◯ ◯ 4 5 ◯ ◯ 4 6 ◯ ◯ 4 7 ◯ ◯ 4Note:◯: excellent,Δ: normal,X: easy separation

Table 4 shows the results of file test, tape test and pencil lead testof a magnesium sample, which is pretreated and then plated with copperunder normal conditions. All the samples of Examples 1˜7 can be seen tohave uniform gloss without color spread.

According to general test procedures, the magnesium alloy sheet having aplating layer was scratched in a 1×1 mm sized lattice form using atungsten blade such that the plating layer was cut along with themagnesium alloy sheet, after which tape was firmly attached to theentire surface of the sheet and then detached therefrom. As the result,no separation was observed.

In addition, a pencil lead test which is used to test the strength ofthe surface was conducted in a manner such that a pencil available fromMitsubishi having hardness of 4H was sharpened and drawn while beingpressed on the surface plated with copper under uniform load. Then, whenthe lead of the pencil was broken without scratches of the surface, thesurface strength was measured. All the samples were passed through thetest. The surface strength was found to be 200H in the presentinvention.

In a file test, the plating sample was vertically cut, held and thenfiled at 45° to the plating surface. While the sample was filed alongwith the plating film, whether or not the plating film was removed fromthe base sheet was measured. The results are shown in Table 4. As shownin Table 4, the samples that underwent the file test were all excellent.TABLE 5 3% NaOH Sample No. Soultion 1 2 3 4 5 6 7 1 day ◯ ◯ ◯ ◯ ◯ ◯ ◯ 2day ◯ ◯ ◯ ◯ ◯ ◯ ◯ 3 day ◯ ◯ ◯ ◯ ◯ ◯ ◯ 4 day ◯ ◯ ◯ ◯ ◯ ◯ ◯ 5 day ◯ ◯ ◯ ◯◯ ◯ ◯Note:◯: excellent corrosion resistanceX: easy corrosion

As is apparent from Table 5, the test samples were colored under normalconditions of the present invention, and dipped into a 3% NaOH aqueoussolution to confirm corrosion resistance. All seven samples wereuncorroded, without any changes in gloss or color. TABLE 6 5% NaClSample No. Soultion 1 2 3 4 5 6 7 1 day ◯ ◯ ◯ ◯ ◯ ◯ ◯ 2 day ◯ ◯ ◯ ◯ ◯ ◯◯ 3 day ◯ ◯ ◯ ◯ ◯ ◯ ◯ 4 day ◯ ◯ ◯ ◯ ◯ ◯ ◯ 5 day ◯ ◯ ◯ ◯ ◯ ◯ ◯Note:◯: excellent corrosion resistanceX: easy corrosion

Table 6 shows the results of corrosion resistance test by dippingsamples into a 5% NaCl aqueous solution. As a result, all seven sampleswere uncorroded, without any changes in gloss or color.

As described above, the present invention provides a method of preparinga copper plating layer having high adhesion to a magnesium alloy throughelectroplating. According to the present invention, after the magnesiumalloy is pretreated for electroplating, a copper (Cu) electroplatingprocess is conducted, thereby obtaining an electrically uniform currentdistribution. In addition, the plating layer having uniform andexcellent adhesion is formed, and thus, the magnesium alloy, which issusceptible to an acid, in particular, an aqueous sodium chloridesolution, has drastically increased corrosion resistance, thereforefurther increasing the usability of the magnesium alloy. Moreover, theadhesion between the pretreated magnesium alloy layer and the copperplating layer can be increased.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A method of preparing a copper plating layer having high adhesion toa magnesium alloy through electroplating, comprising: pretreating themagnesium alloy with a plating pretreatment solution to form a film forelectroplating, serving as a magnesium alloy pretreatment layer,exhibiting a uniform current distribution; and conducting copperelectroplating on the magnesium alloy treatment layer to form the copperplating layer firmly adhering to the magnesium alloy pretreatment layer,in which, upon separation of the copper plating layer by force, thesurface of the magnesium alloy adhering to the copper plating layerexhibits coarse grains contained in the pretreatment layer.
 2. Themethod as set forth in claim 1, wherein the plating pretreatmentsolution comprises 5˜130 g/l of ZnSO₄, 30˜450 g/l of Na₄P₂O₇, 4˜100 g/lof KF, and 2˜100 g/l of Na₂CO₃.
 3. The method as set forth in claim 2,wherein each of K₄P₂O₇ and Na₂CO₃ is used in an amount of about 5˜20 vol% based on a volume of a solution of a dry bath when chemical componentsof the plating pretreatment solution have fatigue due to frequentplating, in order to continuously maintain adhesion between the copperplating layer and magnesium alloy.
 4. The method as set forth in claim1, wherein the plating pretreatment solution comprises 4˜145 g/l ofZnSO₄, 15˜450 g/l of Na₄P₂O₇, 1˜125 g/l of NaF, 1˜125 g/l of Na₂CO₃ and0.5˜45 g/l of KNaC₄H₄O₆, with additives.
 5. The method as set forth inclaim 1, wherein the plating pretreatment solution comprises 5˜80 g/l ofZnSO₄, 4˜380 g/l of K₄P₂O₇, 5˜80 g/l of KF, and 2˜120 g/l of Na₂CO₃. 6.The method as set forth in claim 1, wherein the plating pretreatmentsolution comprises 7˜220 g/l of ZnSO₄, 45˜600 g/l of K₄P₂O₇, 3˜100 g/lof KF, 2˜130 g/l of Na₂CO₃, and 0.5˜58 g/l of KNaC₄H₄O₆, with additives.7. The method as set forth in claim 1, wherein the copper plating layeris formed by sequentially conducting first copper cyanide plating andsecond copper pyrophosphate (CuP₂O₇) plating or third copper sulfateplating, on the magnesium alloy pretreatment layer.
 8. The method as setforth in claim 4 wherein the KNaC₄H₄O₆, added to continuously maintainthe adhesion among the components of the plating pretreatment solution,is used in an amount of 10 vol % or less, due to a sensitivesubstitution reaction, based on the volume of the solution of the drybath.
 9. The method as set forth in claim 2, wherein the copper platinglayer is formed by sequentially conducting first copper cyanide platingand second copper pyrophosphate (CuP₂O₇) plating or third copper sulfateplating, on the magnesium alloy pretreatment layer.
 10. The method asset forth in claim 3, wherein the copper plating layer is formed bysequentially conducting first copper cyanide plating and second copperpyrophosphate (CuP₂O₇) plating or third copper sulfate plating, on themagnesium alloy pretreatment layer.
 11. The method as set forth in claim4, wherein the copper plating layer is formed by sequentially conductingfirst copper cyanide plating and second copper pyrophosphate (CuP₂O₇)plating or third copper sulfate plating, on the magnesium alloypretreatment layer.
 12. The method as set forth in claim 5, wherein thecopper plating layer is formed by sequentially conducting first coppercyanide plating and second copper pyrophosphate (CuP₂O₇) plating orthird copper sulfate plating, on the magnesium alloy pretreatment layer.13. The method as set forth in claim 6, wherein the copper plating layeris formed by sequentially conducting first copper cyanide plating andsecond copper pyrophosphate (CuP₂O₇) plating or third copper sulfateplating, on the magnesium alloy pretreatment layer.
 14. The method asset forth in claim 6, wherein the KNaC₄H₄O₆, added to continuouslymaintain the adhesion among the components of the plating pretreatmentsolution, is used in an amount of 10 vol % or less, due to a sensitivesubstitution reaction, based on the volume of the solution of the drybath.